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    Subject[PATCH 5.7 204/393] powerpc/vdso: Fix vdso cpu truncation
    Date
    From: Milton Miller <miltonm@us.ibm.com>

    [ Upstream commit a9f675f950a07d5c1dbcbb97aabac56f5ed085e3 ]

    The code in vdso_cpu_init that exposes the cpu and numa node to
    userspace via SPRG_VDSO incorrctly masks the cpu to 12 bits. This means
    that any kernel running on a box with more than 4096 threads (NR_CPUS
    advertises a limit of of 8192 cpus) would expose userspace to two cpu
    contexts running at the same time with the same cpu number.

    Note: I'm not aware of any distro shipping a kernel with support for more
    than 4096 threads today, nor of any system image that currently exceeds
    4096 threads. Found via code browsing.

    Fixes: 18ad51dd342a7eb09dbcd059d0b451b616d4dafc ("powerpc: Add VDSO version of getcpu")
    Signed-off-by: Milton Miller <miltonm@us.ibm.com>
    Signed-off-by: Anton Blanchard <anton@linux.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20200715233704.1352257-1-anton@ozlabs.org
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/powerpc/kernel/vdso.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/arch/powerpc/kernel/vdso.c b/arch/powerpc/kernel/vdso.c
    index f38f26e844b64..1c07df1ad560e 100644
    --- a/arch/powerpc/kernel/vdso.c
    +++ b/arch/powerpc/kernel/vdso.c
    @@ -678,7 +678,7 @@ int vdso_getcpu_init(void)
    node = cpu_to_node(cpu);
    WARN_ON_ONCE(node > 0xffff);

    - val = (cpu & 0xfff) | ((node & 0xffff) << 16);
    + val = (cpu & 0xffff) | ((node & 0xffff) << 16);
    mtspr(SPRN_SPRG_VDSO_WRITE, val);
    get_paca()->sprg_vdso = val;

    --
    2.25.1


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